Various SDs, such as diodes and field effect transistors (“FETs”), may be operable to selectively be in an ON or OFF state. The SD may comprise a first electrode and a second electrode that are conductively connectable through an intervening semiconductor component, which has a relatively low resistance when the SD is in the ON state and a relatively high resistance when the SD is in the OFF state. Accordingly, the SD is capable of passing an “ON current” through the first and second electrodes when in the ON state, but not when in the OFF state. For example, where the SD is a diode that is forward biased, the diode is in the ON state and operable to pass the ON current between the first electrode (anode) and the second electrode (cathode) through the intervening and appropriately configured semiconductor component. In another example, where the SD is a FET with an appropriate voltage applied to a third electrode that serves as a gate electrode, the FET is operable to pass the ON current between the first electrode (source) and the second electrode (drain) through the intervening and appropriately configured semiconductor component. The electrodes may be formed on one or more epitaxial layers (“layers”) grown on the substrate. It will be appreciated that a structure that is formed or situated “on” the substrate may be formed on one of the epitaxial layers and not be in direct contact with the substrate itself. Further, the semiconductor components through which the ON current passes may include a portion of the one or more epitaxial layers grown on the semiconductor substrate, with the layers being appropriately configured as needed. For convenience of presentation, “semiconductor substrate” or “substrate”, as used herein, may include the one or more epitaxial layers.
As SDs are reduced in size to achieve higher concentrations of devices having smaller die areas and circuits with smaller cross-sections, current densities through those circuits tend to increase. Current density is commonly expressed as current per cross-sectional area, for example in the unit of Ampere/mm2. In SDs, electrodes are formed as patterned thin metal layers, which may be of substantially uniform thickness. Further, current flow may preferentially occur at or near the surface of the electrodes, especially when the currents are operating under high frequencies. Thus, current density in the context of SD circuits may be expressed as current per cross-sectional width, for example in the unit of Ampere/mm2. The increased current densities tend to make the SDs more susceptible to damage due to Ohmic overheating. Increased current density may also cause electromigration in which a portion of the electrodes is dislodged as a result of momentum transfer to the electrode from conducting electrons, leading eventually to malfunction or failure of the electrodes. In addition, uneven distribution of current through the SD may reduce the overall current that a SD can safely pass because the unevenness creates pockets of overly high current density.